Catalysis

Speeding up the approach to equilibrium

History• Kirchoff in 1814 noted that acids aid hydrolysis of starch to glucose• Faraday (and Davy) studied oxidation catalysts in the 1820’s• Catalyst defined by Berzelius in 1836

A compound, which increases the rate of a chemical reaction, but which is not consumed by the reaction

• Deacon, Messel, Mond, Ostwald, Sebatier processes (HCl, SO2 oxidation, water gas shift, ammonia oxidation, ethene hydrogenation)

• 20th C: ammonia production, cracking reactions, hydrocarbon production, catalytic converters etc.

• Catalysis science developed by Langmuir, Emmett, Rideal and others.

http://dept.chem.polimi.it/~citterio/SilsisMI/Introduction.pdf

Catalysis

When we consider a catalytic reaction, we may imagine that the reaction mechanism consists of many different steps. Catalyst must be a reactant in one of the first steps in the mechanism and a product in one of the last steps.

Heterogeneous catalysis

Chemisorption and catalysis Diffusion of reactants Adsorption Surface diffusion Reaction Desorption Diffusion of products

2 main mechanisms

• Langmuir-Hinshelwood

Reaction between adsorbates

• Eley-Rideal

Reaction between adsorbate and incoming molecule

LH model for unimolecular reaction

B(ads)A (g)A Decomposition occurs uniformly across the surface. Products are weakly bound and rapidly desorbed. The rate determining step (rds) is the surface decomposition step.

Rate = k A

For Langmuir adsorption

p

pk

K1

KRate

pA

fastfast

RDS

khet

A B

LH model for unimolecular reaction

Two limiting cases

High pressures/

Strong binding

Kp>>1

Rate ≈ k

Rate independent of gas pressure

Zero order reaction

Surface coverage almost unity

Low pressures/

Weak binding

Kp<<1

Rate ≈ kKp

Rate linearly dependent on gas pressure

First order reaction

Surface coverage very low

LH model for bimolecular reaction

Langmuir-Hinshelwood reaction with surface reaction as rds

(g) AB (ads) AB (ads) B (ads)A

(ads) B (g) B

(ads)A (g)A

fast

rds

Rate = k AB

pA

fastfastRDS

khet

A

AB

B

pB

Langmuir adsorption of mixed components

BdB

BABaB

AdA

BAAaA

surface

d

a

surfaceg

surface

d

a

surfaceg

kdt

d

pkdt

d

kdt

d

pkdt

d

SBk

kSB

SAk

kSA

rate desorption

)1(B of rate adsorption

rate desorption

)1(A of rate adsorption

Langmuir adsorption of mixed components

Bd

BaSBB

BBdSBBa

Ad

AaSAA

AAdSAAa

k

kp

kpk

k

kp

kpk

BB

AA

K ,K

K ,K

mequilibriuAt

1

1

BAS

BAS

Langmuir adsorption of mixed components

BBAA

BBB

BBAA

AAA

BBAAS

BBAAS

SBBSAAS

pKpK

pK

pKpK

pK

pKpK

pKpK

pKpK

1

1

1

1

11

1

LH model for bimolecular reaction

Rate = k AB

21Rate

BBAA

BBAA

pKpK

pKpkK

LH model for bimolecular reaction

pA

rate

For constant PB

Rate limited bysurface concentration of A

Rate limited bysurface concentration of B

Eley-Ridealbimolecular surface reactions

pA

fast

RDS

khet

AAB

BpBAn adsorbed molecule may

react directly with an impinging gas molecule by a

collisional mechanism

Eley-Ridealbimolecular surface reactions

rate = k pB k KApA pB / (1+KApA)

= 1

pA

rate

For constant PB

Low pressureWeak binding

KApA << 1

rate = khet KA pA pB …….. first order in A

kexp

High pressureStrong binding

KApA >> 1

rate = k pB …….. zero order in A

kexp

Note: For constant pA, the rate is always first order

wrt pB

Diagnosis of mechanism

If we measure the reaction rate as a function of the coverage by A, the rate will initially increase for both mechanisms. Eley-Rideal: rate increases until surface is covered by A. Langmuir-Hinshelwood: rate passes a maximum and ends up at zero, when surface covered by A.

B + S B-Scannot proceed when A blocks all sites.

Transition State Model of Catalyst Activity

reactants

products

Ehom

pote

ntia

l ene

rgy

Eads

Edes

Ehet

reaction co-ordinate

Langmuir-Hinshelwood KineticsAdsorption of reactants and desorption of products

are very fast. Eads and Edes very small.

Surface Reaction is RDS: Ehet

transition state#hom

adsorbed reactants

adsorbed products

#het

Principle of SabatierWhen different metals are used to catalyse the same reaction, it is generally observed that the reaction rate can be correlated with the position of the metal in the periodic table:

A “volcano” curve

Catalyst PreparationFor a catalyst the desired properties are

• high and stable activity • high and stable selectivity • controlled surface area and porosity • good resistance to poisons • good resistance to high temperatures and temperature fluctuations. • high mechanical strength • no uncontrollable hazards

Once a catalyst system has been identified, the parameters in the manufacture of the catalyst are

• If the catalyst should be supported or unsupported. • The shape of the catalyst pellets. The shape (cylinders, rings, spheres,

monoliths) influence the void fraction, the flow and diffusion phenomena and the mechanical strength.

• The size of the catalyst pellets. For a given shape the size influences only the flow and diffusion phenomena, but small pellets are often much easier to prepare.

• Catalyst based on oxides are usually activated by reduction in H2 in the reactor.

Case studies

• Ammonia synthesis (Haber-Bosch)• Hydrogenation of CO (Fischer-Tropsch)

http://www.uyseg.org/greener_industry/index.htm

Ammonia synthesis

A: Steam reformingB: High temperature water-gas shiftC: Low temperature water-gas shiftD: CO2 absorptionE: MethanationF: Ammonia synthesisG: NH3 separation.

Ammonia reactantsSteam reforming

CH4(g) + H2O(g) CO(g) + 3 H2(g)15-40% NiO/low SiO2/Al2O3 catalyst (760-816C)products often called synthesis gas or syngas

Water gas shiftCO(g) + H2O(g) CO2(g) + H2(g)

Cr2O3 and Fe2O3 as catalystcarbon dioxide removed by passing through sodium hydroxide.

CO2(g) + 2 OH-(aq) CO32-(aq) + H2O(l)

Ammonia Synthesis

Fe/K catalyst

exothermic

Mechanism

1 N2(g) + *   N2*

2 N2* + *   2N*

3 N* + H*   NH* + *

4 NH* + H*   NH2* + *

5 NH2* + H*   NH3* + *

6 NH3*   NH3(g) + *

7 H2(g) + 2*   2H*Step 2 is generally rate-limiting. Volcano curve is therefore apparent with d-block metals as catalysts.

Ru and Os are more active catalysts, but iron is used.

Hydrogenation of COHydrogenation of CO is thermodynamically favourable; the first example, methanation catalysed by nickel was reported by Sabatier and Senderens in 1902

CO+3H2CH4+H2O ( G298, -140 kJ/mol)In their classic 1926 papers Fischer and Tropsch showed that linear alkenes and alkanes (as well as some oxygenates) are formed at 200–300°C and atmospheric pressure over Co or Fe catalysts

nCO+(2n+1)H2CnH2n+2+nH2O

2nCO+(n+1)H2=CnH2n+2+nCO2

Since syngas (CO + H2) is readily available from a variety of fossil fuels, including coal, the Fischer–Tropsch process became industrially important for economies which had good supplies of cheap coal but which lacked oil

Fischer-Tropsch

Iron catalysts give mainly linear alkenes and oxygenates, while cobalt gives mostly linear alkanes. Ruthenium, one of the most active catalysts but one which, owing to its expense is little used industrially, can give high molecular weight hydrocarbons; rhodium catalysts make significant amounts of oxygenates in addition to hydrocarbons, while nickel gives mainly methane. Catalyst can be immobilised on Kieselguhr (diatomaceous silicate earth), alumina, active carbon, clays and zeolites.

FT mechanism

adsorption and cleavage of CO and the stepwise hydrogenation of surface carbide giving methylene and other species

Maitlis, P. M.; Quyoum, R.; Long, H. C.; Turner, M. L. Appl. Catal. A: General 1999, 186, 363-374. Towards a Chemical Understanding of the Fischer-Tropsch Reaction: Alkene Formation

Other mechanisms?

• Boudouard reaction. Important in methanation (over Nickel).

2CO C + CO2

Some evidence that hydrogenation of adsorbed carbon leads to formation of hydrocarbons.

Also an important side (undesired) reaction in some hydrocarbon conversion reactions (coking)